There are three kinds of gas absorption spectroscopy using lasers:
(1) DLAS (Direct Laser Absorption Spectroscopy)
(2) WMS (Wavelength Modulated Spectroscopy)
(3) CRDS (Cavity Ring Down Spectroscopy)
In DLAS, laser light is cast into target gas and detected with a photodetector. In this detection, the wavelength of the laser light cast into the gas is fixed at a specific value to measure the amount of absorption by the gas, or a certain range of the laser wavelength is swept to measure an absorption spectrum of the gas. In the former case, the wavelength of the laser light is fixed at an absorption wavelength of the gas and the absorbance at that wavelength is measured. In the case of sweeping the wavelength range, the wavelength of the laser light is varied over a range including the absorption wavelength of the gas to obtain a spectrum of the gas and determine the magnitude of area of the absorption peak formed by the gas (Non Patent Literature 1).
WMS is similar to the wavelength-sweeping mode of DLAS. However, in WMS, not only the wavelength is swept through a range, but the wavelength is also sinusoidally modulated with a cycle sufficiently shorter than the sweeping cycle (i.e. at an adequately high frequency, which is herein denoted by f). The detector is tuned to detect a higher harmonic wave of frequency f (normally, the second-order harmonic wave), whereby the absorption by the gas can be measured with higher sensitivity than by DLAS (Patent Literature 1; Non Patent Literatures 2, 3 and 4). For detection of the higher harmonic wave, lock-in amplifiers are normally used. Another method has also been proposed in which the detector signal is directly subjected to digital sampling and subsequently analyzed by FFT for synchronous detection of 2f (Non Patent Literature 5).
In CRDS, the target gas is placed in an optical resonator composed of at least two mirrors. As one example, a CRDS using a CW (continuous wave) laser is hereinafter described. The light which has entered the optical resonator is reflected and resonated within the optical resonator, and a large part of light with an amount of energy corresponding to the reflectance of the mirrors on both sides is trapped in the resonator. Meanwhile, light with a trace amount of energy leaks to the outside of the mirrors. Accordingly, in the steady state, a stable amount of light energy is constantly stored in the resonator, while a certain amount of light continuously leaks to the outside of the mirrors. In this state, if the laser radiation is discontinued, the light energy in the resonator decays at a rate corresponding to the amount of light lost from the resonator, which simultaneously causes a decay in the intensity of light leaking to the outside of the mirrors. The decay time depends on the amount of light absorbed by the target gas in the resonator. Using this fact, the amount of absorption by the gas is determined. Although this technique is more sensitive than WMS, it is susceptible to the contamination of the resonator, and furthermore, its dynamic range is generally narrow, since the resonator loss rapidly increases with an increase in the amount of absorption, making the measurement impossible. Additionally, a highly nerve-straining control is needed for finely mode-locking the laser in the resonator or in other tasks.
From the previously described facts, WMS is said to be suitable for industrial gas absorption spectroscopic systems due to its favorable balance of sensitivity and robustness (ease of measurement). By WMS, the gas concentration can be easily calculated from the intensity of the obtained absorption spectrum. Additionally, WMS can be used in an application which can measure the concentration and/or temperature of the gas by using two wavelengths even under an environment in which it is impossible to directly measure the pressure or temperature although the temperature and pressure are constantly changing (Non Patent Literature 4).